1. Field of the Invention
The present invention relates generally to the field of bicycles and bicycle control systems used for controlling the transmission of rotational torque generated by a rider to the driving wheel or wheels. More particularly, the present invention relates to a retrofittable, electronic, fully automatic gear shifting control system for multi-gear bicycles.
2. Discussion of the Prior Art
Bicycling is most efficient when the rider maintains a constant number of crank revolutions per minute in spite of changing bicycle speed or changing terrain over which the bicycle is traveling. As used herein, a term "cadence" means the number of crank revolutions per minute or any other cyclic measurement of the rider's activity. A rider generally desires to maintain a cadence that is appropriate for his or her level of physical ability.
In conventional multi-gear bicycles, such as those of a "ten-speed" type, the rider manually changes gears as a function of bicycle speed and difficulty of terrain in order to maintain a comfortable/target cadence. To accomplish this, however, a rider must continuously ascertain what his or her current cadence is and then shift gears as appropriate in order to maintain the preferred cadence. This can be a very subjective process which, as a consequence, is inefficient and imprecise. The rider has to consciously keep track of his or her cadence and shift appropriately, which may be distracting and may adversely affect rider safety. For example, many cyclists do not even know which way to move the shift levers on a multi-gear bicycle in order to increase or decrease their cadence, while some riders find it is difficult to maintain their balance while trying to shift gears because of the requirement of steering with one hand while moving the shift lever(s) with the other hand. In addition, on multi-gear bicycles, the rider must make a small adjustment to the position of the front derailleur whenever the rear derailleur position is changed in order to compensate for the varying chain angle between the rear sprockets and the front chainrings as different gears are selected.
Several prior art systems have been used which attempt to automate the task of shifting the gears. One example of prior art automatic gear shifting systems is the Bike-O-Matic transmission manufactured by Bike-O-Matic, Ltd., of Eagleville, Pennsylvania. In the Bike-O-Matic system, tension placed on the chain due to the rider's pedal effort is used to control shifting. High chain tension causes the derailleur to shift the gears into a lower gear while lower chain tension causes the derailleur to shift the gears into a higher gear. Shifting into a lower gear effectively increases the rider's cadence, while shifting into a higher gear effectively decreases the rider's cadence. One problem with the Bike-O-Matic transmission is that it is a mechanical system that relies on chain tension. During the transition from one gear to another, there is a relatively slow actuation of the derailleur. This slow actuation results in excessive friction caused by rubbing between the chain, derailleur, and sprockets which decreases the rider s efficiency. In addition, when a shift occurs, the mechanism does not position the derailleur in line with sprockets as in the case of manual shifting. The system moves the derailleur continuously rather than discreetly as a function of chain tension. Consequently, the position at which the derailleur stops for a given amount of chain tension may not in fact line up with a particular sprocket on the bicycle.
Furthermore, since the Bike-O-Matic system relies on chain tension, it doesn't help the rider to maintain his or her preferred cadence. For example, on bicycles with multiple front chain rings of different sizes, much greater pedal force would be required to maintain the same chain tension when the rider is using a larger chainring as compared to a smaller chainring. This is because the moment arm of the larger chainring is greater. Using chain tension to control gear shifting can cause shifting at inopportune times, so there could in fact be times when the system will shift gears when the rider does not desire to do so.
Another example of prior art automatic bicycle transmissions is the Browning Electronic Acushift Transmission, manufactured by SunTour. The Browning Transmission electrically shifts the chain using hinged chainring sections. One limitation of the Browning Transmission is that it is designed for use only on the front chainrings of a bicycle. In addition, the Browning Transmission does not use a derailleur, but rather uses a special set of chainrings and electrical controls. Consequently, it is difficult to retrofit this system onto multi-gear bicycles which use conventional derailleurs and chainrings.
Therefore, an object of the present invention is to provide a control system for a human-powered vehicle, such as a bicycle that controls gear shifting so as to maintain a cadence within user-selected limits.
Another object of the invention is to provide a control system for a bicycle that maintains a cadence within user-selected limits over a wide range of bicycle speeds and bicycle gear ratios.
Another object of the present invention is to provide a control system for a bicycle that controls gear shifting automatically so as to maintain a cadence within user-selected limits thereby enhancing rider efficiency and safety.
A further object of the invention is to provide a control system for a bicycle that controls gear shifting automatically and that is retrofittable to a wide range of conventional multi-gear bicycles which use conventional gear shifting systems.